Fluorescein is a man-made chemical that gives off an intense green light when exposed to UV light (like black lights or the Sun).
First created in 1871, we now use it often for research applications because it’s easy to detect. We’ve used it to study the blood-brain barrier and how well certain chemicals get into bones (Source). We also use it in eye drops to find problems with the eye surface (Source).
Long ago, we used to put it into rivers to find out where sewage was being illegally dumped. In 1962, plumbers put it into the Chicago River, and the river turned green. Since then, it’s been a tradition to dye the Chicago River green the weekend of St. Patrick’s Day. Nowadays, though, they use a vegetable dye instead of fluorescein.
Fluorescein is safe enough that we use it in medical applications. As further examples, we use it to determine how well blood is flowing through the blood vessels of your eyes (Source) and during surgery for brain tumors (Source). However, the MSDS (Material Safety Data Sheet) for the substance notes that it can be toxic under certain circumstances, and that the toxicological properties have not been fully investigated. In the case of medical use of fluorescein, we generally agree that the potential benefits outweigh the risks.
The final thing I want to note is that they stopped using fluorescein in the Chicago River because it’s considered to be harmful to the plants and animals of the river (Source)
Fluorescein: Great chemical, as long as we use it responsibly.
Image of fluorescing green fluorescein solution from Wikimedia Commons; Image of green river from elwarren on Flickr; Feature Image of Fluorescein from Steys on Flickr; Inset of cat with eyedrops and picture of fluorescein sodium both from Nottingham Vet School on Flickr; Image of molecule from Wikimedia Commons